BMI provides the most practical measurement of overweight and obesity because it is the same for both sexes and for all ages of adults. When analyzing large populations, BMI is both a valid and reliable tool to estimate overweight and obesity levels.

However, BMI is not a substitute for more precise, objective measurements of body fat composition when examining people on an individual basis. For example, elite athletes with very high levels of lean muscle mass can be mistakenly classified as obese by BMI alone, even though they may have very little body fat. For that reason, calculating BMI is just a starting point, and other body composition assessments should be considered with clients who are on a weight loss journey.